1. Field of the Invention
The present invention relates generally to a method for acquiring a public land mobile network (PLMN) of a mobile station in a cellular communication system. More particularly, the present invention relates to methods for acquiring a PLMN of a mobile station in a frequency overlapping area and providing PLMN information of a base station in a cellular communication system and a base station executing the methods, which enable an acquisition of a home PLMN for registering a mobile station in a home base station that forms an overlapped signal cell together with another base station, i.e., a base station set by a mobile communication operator which provides a mobile communication service to a mobile station, using the same frequency.
2. Description of the Related Art
Conventionally, a mobile station performs a PLMN acquisition procedure in order to register itself in a base station of a corresponding PLMN in an asynchronous cellular communication system. A PLMN acquisition method is a method carried out first during a power-on of the mobile station. The PLMN acquisition procedure includes a slot synchronization acquisition and a frame synchronization acquisition with the base station.
FIG. 6 is a flow chart illustrating a conventional PLMN acquisition method of a mobile station in a cellular communication system. In step P1, the mobile station filters and scans received signals according to preset frequency information in order to sense a carrier frequency having a strongest signal strength. In step P2, the mobile station acquires a slot synchronization for timing-synchronizing a mobile station by using a primary synchronization channel (slot synchronization acquisition step). In step P3, the mobile station acquires a frame synchronization and scrambling code group information by using the slot synchronization acquired in step P2 and a secondary synchronization channel. The frame synchronization is used for acquiring a starting location of a frame. The scrambling code group information is used in a corresponding cell (frame synchronization acquisition step). In step P4, the mobile station acquires scrambling code information to be used in the corresponding cell using the scrambling code group information acquired in step P3 and a common pilot channel (scrambling code acquisition step). In step P5, the mobile station demodulates a primary common control physical channel using the scrambling code information acquired in step P4 and receives system information to be broadcasted in the corresponding cell.
In step P6, the mobile station extracts PLMN information included in the system information received in step P5. The PLMN information is business operator identification information. In step P7, the mobile station determines whether the received PLMN information is home PLMN information based on the extracted PLMN information (PLMN information acquisition step), i.e., whether a Mobile Country Code (MCC) and Mobile Network Code (MNC) of the PLMN identity are the same as the MMC and MNC of the International Mobile Subscriber Identity (IMSI). It is assumed that a user automatically selects a PLMN of the mobile station. When the received PLMN information is the home PLMN information, the mobile station registers itself in a base station that transmitted the PLMN information in step P10. When the received PLMN information is not the home PLMN information, the mobile station stores the received PLMN information in available PLMN information in step P8. That is, the mobile station stores the received PLMN information in a PLMN information memory to which the PLMN information is to be stored. In step P9, the mobile station changes a carrier frequency in order to search for the home PLMN. Then the routine returns to step P2.
However, in an overlapped area in which a plurality of business operators provide a mobile service using an identical frequency, the mobile station has a problem in that steps P3 to P10 are performed only for a slot synchronization having a strongest receiving strength in step P2. This problem will be described below in detail with reference to FIG. 3.
In FIG. 3, although a secondary slot synchronization signal is a signal for a home PLMN, the second slot synchronization signal is slightly lower in strength than the first slot synchronization signal. Accordingly, because steps P3 to P5 for the second slot synchronization are not executed on the second signal because it has a lower signal strength, the mobile station cannot receive system information regarding the second slot synchronization signal. As a result, in a situation such as this, the mobile station does receive a home PLMN service in an area where home PLMN service is available. In such a case, the mobile station registers in a different PLMN in order to receive the mobile service. However, when the different PLMN does not provide the mobile service, the mobile station cannot receive the mobile service.